Mass spectrometry has proven to be an effective analytical technique for identifying unknown compounds and for determining the precise mass of known compounds. Advantageously, compounds can be detected or analysed in minute quantities allowing compounds to be identified at very low concentrations in chemically complex mixtures. Not surprisingly, mass spectrometry has found practical application in medicine, pharmacology, food sciences, semi-conductor manufacturing, environmental sciences, security, and many other fields.
A typical mass spectrometer includes an ion source that ionizes particles of interest. The ions are passed to an analyser region, where they are separated according to their mass (m)-to-charge (z) ratios (m/z). The separated ions are detected at a detector. A signal from the detector may be sent to a computing or similar device where the m/z ratios may be stored together with their relative abundance for presentation in the format of a m/z spectrum. Mass spectrometers are discussed generally in P. H. Dawson, Quadrupole Mass Spectrometry, 1976, Elsevier Scientific Publishing, Amsterdam.
An ion guide guides ionized particles between the ion source and the analyser/detector. The primary role of the ion guide is to transport the ions toward the low pressure analyser region of the spectrometer. Many known mass spectrometers produce ionized particles at high pressure, and require multiple stages of pumping with multiple pressure regions in order to reduce the pressure of the analyser region in a cost-effective manner. Typically, an associated ion guide transports ions through these various pressure regions.
A collision cell is a particular form of an ion guide that forms part of the analyser region, to improve the analysis of a sample. Collision cells fragment “parent” or precursor ions as a result of energetic collisions. They consist of a pressurized container (such as a ceramic or metal cylinder); gas (typically N2 or Ar, pressurized from 0.1 to 10 mTorr); and the ion guide.
Ions may be fragmented when they are accelerated into the pressurized gas with sufficient kinetic energy. The collision cell must effectively capture these fragment ions, contain them along an axis, and transport them to the exit of the collision cell. A collision cell should guide and capture fragment ions and transports them with high efficiency.
Most ion guides and collision cells include parallel ion guide rods, often arranged in sets of two, three or four rod pairs. RF voltages of opposite phases are applied to opposing pairs of the rods to generate an electric field that contains the ions as they are transported in a gaseous medium from the entrance to the exit. An axial field may be used to accelerate ions within the ion guide, for example for fragmentation, and then to move ions along from the entrance to the exit. The axial field is significant as ions tend to slow down almost to a halt without it.
The axial field may, for example, be produced by manipulating the shape of the field produced by the parallel rods. The relative voltages on the neighboring rods determine the axial field. Unfortunately, ion guides that rely on the shape of the electric field between the rods to produce an axial field tend to distort the electric field asymmetrically, reducing mass range and sensitivity.
Other known ion guides use auxiliary electrodes in conjunction with the guide rods to produce an axial electric field. A DC voltage is applied to the auxiliary electrodes that, in conjunction with the rod set, serve to produce an axial field.
Unfortunately, the use of auxiliary electrodes tends to be complex and expensive. For example, for 2n guide rods in the ion guide, there will be 2n auxiliary rods, giving a total of 4n rods, increasing cost and complexity substantially.
Accordingly, there remains a need for a low cost and low complexity ion guide and collision cell that provides an axial field.